Purpose of Cowl Bumps

[JoeRuscito]

The manual says optional. Do they provide clearance for a part of the engine? Im installing the 912is. It seems some install and some do not.

[efwd]

I simply installed for the looks. I look into the cowl and note that the clearance is very close but likely doable without. Nice thing is, you can get the install of the cowl completed and add them anytime before paint.

[896tr]

The purpose of the bump cowl is to give the plane the look of having a radial engine, some like it others like the smooth look better, It is just an engine cover with drag reducing qualities either way. That's my opinion and I'm sticking to it.

[jrevens]

The purpose of the bump cowl is to give the plane the look of having a radial engine, some like it others like the smooth look better, It is just an engine cover with drag reducing qualities either way. That's my opinion and I'm sticking to it.

896tr - I believe they are talking about the smooth cowl, not the traditional "radial-look" round one. Sometimes "bumps" are needed for engine or accessory clearance with engines other than the standard 912. As far as drag-reducing qualities go, the smooth cowl is definitely less "draggy" and more aerodynamic than the round one.

[JoeRuscito]

896tr - I believe they are talking about the smooth cowl, not the traditional "radial-look" round one. Sometimes "bumps" are needed for engine or accessory clearance with engines other than the standard 912. As far as drag-reducing qualities go, the smooth cowl is definitely less "draggy" and more aerodynamic than the round one.

Correct, referring to the new smooth cowl. I like the smooth look, just looking ahead and wondering if there was a clearance issue.

[mike92104]

From Wikipedia:

"The design was originally intended to use a new radial engine then in development and the early Kitfoxes had round cowls with bumps to accommodate the cylinder heads. Although this radial engine did not materialize, and a Rotaxtwo-stroke engine was adapted instead, the "retro" radial cowling proved popular and was retained on many models."

Mike

[efwd]

Since you have decided these are probably not needed but here is the pic I promised. Looks to me that you would be fine without.

[efwd]

Joe, I will see if I can get a good pic of the inside clearance of the smooth engine cowl with the bump installed. It is the fuel rail for the fuel injection that gets very close to the top cowl. When I built mine, I seemed to have everything lined up but gravity pulls my cowl down about 1/4 inch at the prop spinner such that I can see the back side of the spinner a little bit. Back at the fitting of the cowl bump I seem to recall I had about 1/4 inch of space between the cowl and that rail fitting. I am not certain that the cowl would not rest on that rail if my bumps weren't installed given the fact that gravity pulls my cowl down a bit.

[JoeRuscito]

Joe, I will see if I can get a good pic of the inside clearance of the smooth engine cowl with the bump installed. It is the fuel rail for the fuel injection that gets very close to the top cowl. When I built mine, I seemed to have everything lined up but gravity pulls my cowl down about 1/4 inch at the prop spinner such that I can see the back side of the spinner a little bit. Back at the fitting of the cowl bump I seem to recall I had about 1/4 inch of space between the cowl and that rail fitting. I am not certain that the cowl would not rest on that rail if my bumps weren't installed given the fact that gravity pulls my cowl down a bit.

Thanks for the explanation. Pictures would be awesome! Ill have mine on soon enough and I will keep in mind the sag and try to offset.